Fixed CSS issues:
* In the tag input element, when entering a new tag,
the "Add ..." item was missing padding. Padding added.
* In the timezone selects the caret was hidden, so it was
hard to discover that timezone options can be filtered by typing.
Caret in timezone selects is now visible and blinking.
The form controls with the .form-control CSS class have
background: transparent (as defined in Bootstrap's variables.less).
On Chrome, in dark mode, this causes <select> dropdowns to have
light text on light background. As a workaround, we set the
select.form-control background in base.css to the same color as panel
background.
* Add Check.last_start_rid field
* Fill Check.last_start_rid on every start event
* Clear Check.last_start on every "fail" event
* Clear Check.last_start on success event if either case is true:
- the event's rid matches Check.last_start_rid
- the event does not specify rid
In human terms, the alerting logic will be: we track the
execution time of the most recent "start" event only. It would
take a major redesign to track the execution time of all
concurrent "start" events and send alerts when *any* of them
overshoots the time budget. So, whenever we see a "start" event,
the timer resets.
Example:
* 00:00 client sends start signal with rid=A, timer starts
* 00:10 client sends start signal with rid=B, timer resets
* 00:20 client sends success signal with rid=A, timer
does not reset because rid A does not match the rid seen in
the most recent start signal (it was B)
* 00:30 the grace time runs out, the check's status shows
as started + failed
At this point the check can be reset to a healthy state in 3
different ways:
* send a success signal with rid=B
* send a failure signal with any rid value or without it
* send a success signal without a rid value
* Added duration to ping details. This is useful on a device with a small screen, since the duration cannot be seen in the main view so now one can see it in the ping's details.
* Changed terms across the board from "delta" to "duration"
* timedelta is now consistently imported as "td" across the entire project (even in Django generated migration files)
