Plugin install endpoint: adding an endpoint to install a plugin
Uniform Jetpack Endpoints.

+ Improve base class.
+ Add $expected_actions property
+ Remove unused functions
Add bulk install capacity.

Needs some testing, and tests :)
Add modules bulk activation/deactivation logic.
Ooops
Return the non ulrencoded version as id.
Add test case for plugin installation endpoint.
Moved dependencies out of the install function.
Fix typo in plugin download links accessing.
Add return value for plugin installation.
Use a custom get request instead of plugins_api

It looks like `plugins_api` doesn't work with slugs like `the/the.php`,
only `the`. Let's use a custom request intead where those slugs work
just fine.
Remove unneeded dependency in install endpoint.
Plugin install endpoint: adding an endpoint to install a plugin
Add bulk install capacity.

Needs some testing, and tests :)
Add test case for plugin installation endpoint.
Moved dependencies out of the install function.
Fix typo in plugin download links accessing.
Add return value for plugin installation.
Use a custom get request instead of plugins_api

It looks like `plugins_api` doesn't work with slugs like `the/the.php`,
only `the`. Let's use a custom request intead where those slugs work
just fine.
Remove unneeded dependency in install endpoint.