Gitea: Add Issue and Pull Request templates

It is a part of the Phabricator to Gitea migration.

The issue template is based on the bug submission instructions which
are shown in the Phabricator's bug submission form. Some further tweaks
are likely needed, but the current version of the template simplifies
re-iteration while working on the migration.

The pull request template is needed to override the template in the
.github folder which is otherwise picked up by Gitea.

.gitea/issue_template.yaml

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+name: Bug Report
+about: File a bug report
+labels:
+  - bug
+ref: master
+body:
+  - type: markdown
+    attributes:
+      value: |
+                ### First time bug reporting?
+                Read [these tips](https://wiki.blender.org/wiki/Process/Bug_Reports) and watch this **[How to Report a Bug](https://www.youtube.com/watch?v=JTD0OJq_rF4)** video to make a complete, valid bug report. Remember to write your bug report in **English**.
+
+                ### What not to report here
+                For feature requests, feedback, questions or issues building Blender, see [communication channels](https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests).
+
+                ### Please verify
+                * Always test with the latest official release from [blender.org](https://www.blender.org/) and daily build from [builder.blender.org](https://builder.blender.org/).
+                * Please use `Help > Report a Bug` in Blender to automatically fill system information and exact Blender version.
+                * Test [previous Blender versions](https://download.blender.org/release/) to find the latest version that was working as expected.
+                * Find steps to redo the bug consistently without any non-official add-ons, and include a **small and simple .blend file** to demonstrate the bug.
+                * If there are multiple bugs, make multiple bug reports.
+                * Sometimes, driver or software upgrades cause problems. On Windows, try a clean install of the graphics drivers.
+
+                ### Help the developers
+                Bug fixing is important, the developers will handle a report swiftly. For that reason, we need your help to carefully provide instructions that others can follow quickly. You do your half of the work, then we do our half!
+
+                If a report is tagged with Needs Information from User and it has no reply after a week, we will assume the issue is gone and close the report.
+
+  - type: textarea
+    attributes:
+      label: "Description"
+      value: |
+               **System Information**
+               Operating system:
+               Graphics card:
+
+               **Blender Version**
+               Broken: (example: 2.80, edbf15d3c044, master, 2018-11-28, as found on the splash screen)
+               Worked: (newest version of Blender that worked as expected)
+
+               **Short description of error**
+
+               **Exact steps for others to reproduce the error**
+               Based on the default startup or an attached .blend file (as simple as possible).
+

.gitea/pull_request_template.md

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+---
+name: Pull Request
+about: Submit a pull request
+---