Basic skeleton for repository customization

This commit refs #3897 and #3898.
Rename widget class
Add two function helpers to deal with repo logo
Show editTheme link only if available
Update form so the user can upload the repo logo
Add jquery-miniColors
jquery-miniColors trigger for .color-picker
Add jquery-miniColors assets to editTheme
More progress in the editTheme form

banner and banner_delete are added now with the same behavior
that logo and logo_delete had before. content has been renamed
as htmlSnippet. Internally, QubitRepository handles these new
proporties as QubitProperty records.
Add jQuery miniColors to COPYRIGHT
Small fixes

Invoke save() method when we are done with the repository object.
Also, don't force to store properties with sourceCulture enabled,
only when the field is not translated (like background).
s/background/backgroundColor
Use miniColors only if js was loaded
Render backgroundColor and htmlSnippet repository settings
Validate hex colors
Customize validatedFile class to crop banner/logo
Crop logo/banner with mongrify if available
Add help text below banner/logo fields
Render banner with img tag
Use wildcards in i18n labels