New announcement system for TheyWorkForYou

[ajparsons]

This PR adds a more general announcement feature to TheyWorkForYou.

• This extends the current Banner approach to allow multiple banners to rotate at once.
• New 'announcements' can be made in three locations: the homepage, MP sidebar, and a donation box in the MP sidebar.

The three locations style content in slightly different ways:

• 'homepage' - uses all the content, but is quite wide rather than in a sidebar.
• 'sidebar' - uses image, title and button options, but does not display the additional 'content' option.
• 'donation' - uses title, content and button options (no image).

The backend has been adapted to store JSON that stores details of multiple options. Format is shown here. Invalid Json throws an error (but is not checked against a specific schema):

• Announcements https://gist.github.com/ajparsons/ff8029afd333122775864b58fbe29433
• Banners https://gist.github.com/ajparsons/778e752ac110f595e1230ee908be9c5e

Dev helpers:

For consistency with the script-to-rule-them-all approach elsewhere the 'script' directory as opposed to 'script', stores several useful management commands for use in dev. Currently this contains watch-css and `create-superuser'.

Other styling changes:

• The announcement section on homepage has demoted the 'featured' debate to below the search bar.
• A new green 'Browse content' helps draw attention to the sidebar on MPs profiles.

[ajparsons]

Things not covered in this PR:

• This doesn't cover - but probably should - adding announcements to alerts. Will do this as a future PR because testing it is slightly different.

[ajparsons]

Should have fixed all of the big things and it looks like it works okay.

If seems alright, I'll fix up the history to something sensible.

[ajparsons]

OK! Should be almost there.

[ajparsons]

Okay! History tidied up (and spelling fixed). Will merge in this afternoon!