Implement tags

[Changaco]

Reticketed from #66.

• add a tags column to the statements table
• add an <input> below the statement <textarea> on the profile edit page
• update the tags when the user updates a profile statement
• figure out how to index the tags so we can query them efficiently
• implement tags lookup in the search page
• render links, below profile statements, as links to the search page

Extra:

• add auto-completion to the <input>
• implement a "leader-board" mode for the search page so we can view tags like the old Gratipay communities