Editor links, Markdown attributes and general improvements

Features:

• Editor links have been added to the visual editor to allow customisation of your interfaces.
• Commit hashes are available on each site activity from GitHub and Bitbucket.

Fixes:

• srcset and sizes attributes are now valid attributes within the Markdown editor. These were previously filtered out on save.
• Renaming folders to the same name with different cases was causing odd behaviour.
• Prevented multiple error messages on upload failure.
• Removing i18n from a site would cause the site to get stuck with the previous i18n.
• Bitbucket initial syncing got stuck in a particular state.