New flakiness dashboard should support substring matching

New flakiness dashboard should support substring matching
https://bugs.webkit.org/show_bug.cgi?id=123393

Reviewed by Alexey Proskuryakov.

Addressed the use cases by
1. Always showing the candidate even when there is exactly one test matching the current value.
2. Adding all tests that match the current value upon the user pressing enter key.

* public/index.html:
(fetchManifest): Add all tests that match the current value. Confirm whether the user really
want to add all the tests when there are more than 15 tests to add.
* public/js/autocompleter.js:
(Autocompleter.prototype.filterCandidates): Extracted from _updateCandidates.
(Autocompleter.prototype._updateCandidates): Show the candidate window even when there is
exactly one test that matches the criteria so that the user can select this test.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@158094 268f45cc-cd09-0410-ab3c-d52691b4dbfc