When updating an expense, re-apply filters

Updating an expense asks the preferences view to filter again, thus triggering
the filtering in the expenses index view.
Expense form submit button changes text depending on whether the expense is a
new record or not.

TODO:
Add specs for this functionality
Filter by current month on page load

No tests again...
Reset filters functionality

Set time filter to 'all time' and clear tags
Re-apply filters when adding an expense

Expense add triggers expenses:changed event
Reports page. Expenses by tag and by month

Pie chart for tags and drilldown for months. The drilldown is an analysis
of expenses by tag for the specific month.

TODO:
- unique tags bug for tags 'a,b' and 'b,a'
- no tests
Fixed bug with unique tags

Sort expense's tag_list so tags 'a,b' and 'b,a' are the same. Filter collection
by exact tags.
Differentiate expenses filtering by tag in filters and reports

Filters is done when expense tag_list contains a tag. Reports calculate
totals by exact tag_list match