squash! Adding issue template for bugs.

Adding issue and merge request templates

.gitlab/merge_request_templates/default.md

+## Merge Info
+
+<!-- Bookkeeping information for release management -->
+
+- ### Related Issues
+  - tor-browser#xxxxx
+  - tor-browser-build#xxxxx
+  - etc
+
+- ### Backport Timeline
+  - [ ] **Immediate** - patchsets for critical bug fixes or other major blocker (e.g. fixes for a 0-day exploit) OR patchsets with trivial changes which do not need testing (e.g. fixes for typos or fixes easily verified in a local developer build)
+  - [ ] **Next Minor Stable Release** - patchset that needs to be verified in nightly before backport
+  - [ ] **Eventually** - patchset that needs to be verified in alpha before backport
+  - [ ] **No Backport** - patchset for the next major stable
+
+- ### Upstream Merging
+  - [ ] Merge to `base-browser` - typically for `!fixups` to patches in the `base-browser` branch, though sometimes new patches as well
+    - **NOTE**: if your changeset includes patches to both `base-browser` and `tor-browser` please please make separate merge requests for each part
+
+- ### Issue Tracking
+  - [ ] Link resolved issues with appropriate [Release Prep issue](https://gitlab.torproject.org/groups/tpo/applications/-/issues/?sort=updated_desc&state=opened&label_name%5B%5D=Release%20Prep&first_page_size=20) for changelog generation
+
+## Change Description
+
+<!-- Whatever context the reviewer needs to effectively review the patchset -->
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