Fix multisite bugs & enhance functionality

This PR should fix issue #935.

Bugs fixed (were introduced by v1.5):
- saving of settings on the multisite settings page was not working.
- restoring site to default settings from multisite settings page was not working.
- initializing new blogs with settings from a chosen default blog was not working (might still not be completely stable for WP multisite with WPSEO in must-use plugins directory, stable in all other cases).
- wrong option debug information shown on multisite settings page

Enhancements:
- When using the 'Reset default settings' button on a blog in a network while another blog has been chosen to be used as a basis for the settings for all new blogs, the reset will respect that setting and reset the blog to the settings from the chosen blog.
- For small networks ( < 100 sites ), the network page user interface has been improved by offering drop-down lists of the blogs for blog selection fields. For larger networks, the interface remains the same (i.e. text fields expecting blog id input).

Also:
- Added a few extra options to the network blog reset/copy exclude list (theme dependent ones), so new blogs won't be populated with values which are not valid for their site.
- clean up wrongly stored options (i.e. wpseo_ms in options table not in sitemeta table) = fall-out of the above bugs
- made the padding on select boxes slightly smaller as letters were being cut off at the bottom.

Screenshot of the enhanced network admin page for small networks:
[**removed** - see new screenshot below]