Add tag support and refactor history.

This commit introduces basic (database/model) support for tagging and
cleans up a bunch of historical oddities. Numerous queries and functions
have been removed.

As an unfortunate result of this, a database change is required.
Specifically, the 'locked' column becomes a non-null integer and a new
column, 'tag' is added. The best way to apply these changes is to drop
(or rename) your history table (migration is up to you).
Add marking (like VIm).

Clip now supports the ability to tag/mark an entry for quick selection.
To mark a selected record, press 'm' and then the desired mark letter.
For example, pressing 'ma' will mark the record with the tag, 'a'.

To activate a marked record, press the gave character, '`', followed by
the desired mark. For example, to select the previously-selected 'a'
record, press '`a'. If the mark is found, the correlated record will be
activated.