Fix opening a /blob/ URL using combined repository list/URL clone dialog

[jcansdale]

What this PR does

Entering a repository or /blob/ URL into the filter box should allow the repository to be cloned or opened.

• Light up Clone or Open button when a valid URL is entered into filter box
• Deactivate Clone and Open buttons when URL is no longer valid

How to test

Allow /blob/ URLs

1. Open Open from GitHub dialog
2. Paste a blob URL into filter (e.g https://github.com/github/VisualStudio/blob/master/README.md)
3. Clone or Open button should activate

Deactivate buttons when URL is invalid

1. Open Open from GitHub dialog
2. Paste a repository URL into filter (e.g https://github.com/foo/bar)
3. Clone or Open button should activate
4. Delete the end of URL until it's no longer valid (e.g https://github.com/foo/)
5. Clone or Open button should become inactive

Fixes #2214