chore: clean up issue templates

[paulbalandan]

Description
The 2nd and 3rd latest active issues we have are created not following the bug report template, which are essentially feature requests. This is in contradiction that Github issues are to be used for bug reports while feature requests and/or support questions are made first in the forums. This PR fixes that.

• Delete the feature_request.md
• Add wordings in the config.yml that feature requests should be handled first in the forums.
• Retain the planned_work.md for maintainers' space (as it should be).
• Add's the mergeable GitHub app config file to enforce above rules. If a "bug" report not following the template is made (i.e., uses the planned work template) it will close the issue promptly. Exceptions are maintainers so they can use that space.

Should this PR be merged, next step would be to install the mergeable app to this repo.
Documentation: https://mergeable.readthedocs.io/en/latest/index.html
To install: https://github.com/apps/mergeable

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

That makes sense. I'll have to try to visit the forum more often.