[MM-611]: Added the feature to select the default repository for the channel

[Kshitij-Katiyar]

Summary

Created the slash commands /github default-command get, set, and unset, to handle the default repo for channel per user. The default repo will be auto-filled in the create GitHub issue modal so the user won't need to search the repo everytime.

Ticket Link

Fixes #611

What to tests

1. Run the commands /github default-command get, set, and unset and see if they are working properly or not.
2. After setting the default repo, run the /github issue create command, it should open a modal with the default repo autoselected.

[wiggin77]

LGTM 👍

[arush-vashishtha]

@Kshitij-Katiyar, I have tested this PR, and the slash commands to set, get, and unset the default repo are working fine. However, the issue create modal does not open upon using the " /GitHub issue create " command in the channels.

[Kshitij-Katiyar]

@Kshitij-Katiyar, I have tested this PR, and the slash commands to set, get, and unset the default repo are working fine. However, the issue create modal does not open upon using the " /GitHub issue create " command in the channels.

@arush-vashishtha the issue has been fixed, please re-test the PR.

[AayushChaudhary0001]

This PR has been tested for the code changes and the slash command to set, un-set and get the default repository set for the channel is working fine. Also, after setting the default repo, the modal opened for creating any issue shows the already repository in the modal, LGTM. Approved.