Change labels and default title applied to new issues. (#5013)

<!-- READ ME FIRST: Please fill in the explanation section below and
check off every point from the Essential Checklist! -->
## 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](https://github.com/oppia/oppia/wiki/Triaging-process) 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
<!-- Please tick the relevant boxes by putting an "x" in them. -->
- [ ] 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: ...".)
- [x] Any changes to
[scripts/assets](https://github.com/oppia/oppia-android/tree/develop/scripts/assets)
files have their rationale included in the PR explanation.
- [x] The PR follows the [style
guide](https://github.com/oppia/oppia-android/wiki/Coding-style-guide).
- [x] The PR does not contain any unnecessary code changes from Android
Studio
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#undo-unnecessary-changes)).
- [x] The PR is made from a branch that's **not** called "develop" and
is up-to-date with "develop".
- [x] The PR is **assigned** to the appropriate reviewers
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#clarification-regarding-assignees-and-reviewers-section)).

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,8 +1,8 @@
 ---
 name: Bug report
 about: Create a report to help us improve
-title: ''
-labels: 'Type: Bug, Status: Not started'
+title: '[BUG]: '
+labels: 'triage needed, bug'
 assignees: ''
 
 ---

.github/ISSUE_TEMPLATE/feature_request.md

@@ -1,8 +1,8 @@
 ---
 name: Feature request
 about: Suggest an idea for this project
-title: ''
-labels: 'Type: Improvement, Status: Not started'
+title: '[Feature Request]: '
+labels: 'triage needed, enhancement'
 assignees: ''
 
 ---