Add and populate Automatic updates column, add and handle enable/disable auto-updates bulk actions to the multisite themes list table.

[pbiron]

Fixes #15.

A few notes:

0. I put this together by copy-paste-modify the equivalent functions for plugins. Please double check that I correctly replaced all occurrences of plugin with theme (including in comments, etc).
1. While the hooks exist to add the Auto-update enabled/disabled views to the list table, there is not core hook equivalent to the one we use for the plugins list table to actually restrict the themes shown in the list according to those views. Therefore, this PR does not added those views
   • adding those views when this plugin is merged into core will be no problem
   • we should call this out in the changelog when 0.4.0 is released
2. I realized when putting this together that we are not currently displaying admin notices when bulk actions are applied for plugins. Therefore, I didn't do them for this PR either. I'll open an issue (and do a separate PR) for that.
3. I just realized (while typing this) that I forgot to cover the case of the list table in Network > Sites > Edit > Themes. I'll update this PR to cover that shortly.

[bookdude13]

Themes, not plugins :)

[pbiron]

see, I knew I'd probably missed it somewhere :-). Thanks!!

[pbiron]

4. I just realized (while typing this) that I forgot to cover the case of the list table in Network > Sites > Edit > Themes. I'll update this PR to cover that shortly.

That screen should now be covered.

@audrasjb Supporting themes in both of the multisite screens complicated the code a little bit, since both the "action" and "redirect" URLs need to account for both screens. The code could probably use a refactor to make it cleaner, but I think it's good enough to merge and do the refactor separately once the functionality is confirmed to work (there are some other parts unrelated to this PR that I'd like to refactor as well), .

[pbiron]

3. I realized when putting this together that we are not currently displaying admin notices when bulk actions are applied for plugins. Therefore, I didn't do them for this PR either. I'll open an issue (and do a separate PR) for that.

Turns out that admin notices were correctly showing on non-network screens, and the reason admin notices weren't being show on the network screens is that another hook is needed for that. The recent commit adds that hook...and fixes another copy-past typo.

[audrasjb]

Great job @pbiron thanks!
Looks good to me, let's merge this one now :)

[audrasjb]

good to go