Change labels and default title applied to new issues.

[seanlip]

Explanation

Update the github configuration so that new issues receive the labels "triage needed" and "bug", and new feature requests receive the labels "triage needed" and "enhancement". This mirrors the triaging system used on Oppia Web, and in fact most of the current labels in the existing GitHub configuration are obsolete.

Also updates the default issue titles in the same way as Oppia Web.

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

[seanlip]

@adhiamboperes PTAL. Thanks!

[adhiamboperes]

@seanlip these changes LGTM.

[oppiabot]

Unassigning @adhiamboperes since they have already approved the PR.

[oppiabot]

Hi @seanlip, this PR is ready to be merged. Please address any remaining comments prior to merging, and feel free to merge this PR once the CI checks pass and you're happy with it. Thanks!