Make use of goals' tags

[krugerk]

The goal resource now includes a list of a goal's tags. Eventually the iOS app may want to support these.

Ideas from krugerk:

• show the tags across the goal cell in the gallery
• support grouping goals by tags
  • See Altbee for an example. Altbee allows one to create goal categories based on tags. These categories can be sorted and appear as such. It uses a fallback category containing all red goals as well as all others not included in one of the other categories (tags).
• support filtering by tags
• maybe tapping a tag can work as a quick filter to quickly filter goals with just those tags, perhaps updating the search bar accordingly
• support grouping based on combination of tags (goals may contain more than one tag)
  • support primarily AND
  • provide support though for OR, when specifying more than one tag
• widget (currently today extension) should be configurable to not care about tags, only allow goals with certain tags to be shown, prevent goals with certain tags from being shown

[dreeves]

Step 0 before we think about this one too hard will be having a reasonable interface for goal tags on the website.