Add help tabs on update-core, plugins, and themes admin screens

[audrasjb]

Fixes #105

This PR adds help tabs on update-core, plugins, and themes admin screens.
It also handles network screens.

See screenshots below:

update-core.php

themes.php

plugins.php

Important: this PR needs copy review

Here are the current strings:

update-core.php

Plugins and Themes with auto-updates enabled will display the estimated date of the next auto-update. Auto-updates schedule depends on WordPress planned tasks.
Please note: WordPress auto-updates may be overridden by third-parties plugins or custom code.
For more information: {LINK}Documentation about auto-updates{LINK}

themes.php

Auto-updates can be enabled and disabled on a theme by theme basis. Themes with auto-updates enabled will display the estimated date of the next auto-update. Auto-updates schedule depends on WordPress planned tasks.
Please note: WordPress auto-updates may be overridden by third-parties plugins or custom code.
For more information: {LINK}Documentation about auto-updates{LINK}

plugins.php

Auto-updates can be enabled and disabled on a plugin by plugin basis. Plugins with auto-updates enabled will display the estimated date of the next auto-update. Auto-updates schedule depends on WordPress planned tasks.
Please note: WordPress auto-updates may be overridden by third-parties plugins or custom code.
For more information: {LINK}Documentation about auto-updates{LINK}

[pbiron]

Regarding copy editing: do you think there is time before 0.7.1 is ready to be released to get at least a first pass done on that review? If not, would it be OK to add IMPORTANT: This needs copy review to the content and merge?