Allow customisation of manifest

[dracos]

Depends on #2220

Allow bits of the manifest to be customisable, e.g. the theme colour, app logos, and so on.
This could also be used/expanded for e.g. favicon, HTML theme colour, and then to being able to customise basic logo/styling in admin itself, though having non-static stylesheets more of a challenge!

For starters, going by #2220 work (see manifest commit on pwa branch), an admin page that lets you set, for a cobrand:

• name
• short_name
• theme_colour
• background_colour
• Icons (192 and 512 are defaults, but perhaps any square multiple of 48), saved in web/theme/*moniker*/icons/ maybe?

I'd guess a theme database table with cobrand/key/value columns? Or cobrand and those things as columns if we don't think it's going to extend a lot...

Then that'd be picked up for the manifest. After that exists, it could set the theme colour in the HTML head (as fixmystreet.com cobrand does manually at present), and also then favicon would be next step, new ticket by then though I'd say!