Add wp-admin menu for git repository configuration

[madhusudhand]

This adds Admin menu for git repository configuration.

It enables to add settings such as Repository URL, Access Token etc.. and Disconnect repository.
It saves the set configuration using 'update_option'.

Note: This just saves and deletes the settings. Does not clone the actual repository.

[pbking]

Considering how in previous incarnations of this work, from the site editor there was the option to connect individual themes to a repository. This (from a UX perspective at least) seems to connect the entire SITE to a single repository.

If we're going to keep the repository settings per-theme then I believe there should be two options for this:

• Connect THIS theme to a repository
• Install a theme from a repository

Does that line up with your thinking?

[madhusudhand]

@pbking I created this as small chunk to speedup the review process and bring the changes in :)
But I agree, having both options (connecting with all themes, and active theme) should give the overall picture for review.

Pushed next set of changes in this PR and following are few screenshots.

[pbking]

We no longer have plans for CBT to manage a theme via git so closing this as an "interesting experiment".