Add new feature to Show Last: X

Where X is one of:

- 30 Minutes
- 1 Hour
- 2 Hours
- 12 Hours
- 24 Hours
- 1 Week
- 2 Weeks
- 30 Days
Handle submitted value, allow custom intervals

- Form will dynamically clear unecessary values (e.g. date_start when
  using a limit)
- Search results are now correctly limited
- Add a new input for custom intervals based on DateInterval
Minor fixes based on feedback from @markstory
Merge pull request #69 from dshafik/limit-to-last-x

Add new feature to Show Last: X