* Reopen log files after receiving a SIGHUP signal

Implements a system signals listener in log.FileLogWriter.

After receiving a SIGHUP signal from the system, the listener will close the current log file and then open it again.

The listener will finishes after receiving a SIGINT, SIGKILL or SIGTERM signal.

Closes grafana/grafana#2497

* Move the SIGHUP handling to the main file

response.ContentLength might be invalid if the http response
is chunked.

fixes #13079

[skip ci]

Alerting notification reminder

* updated rules for variable name and fixed files that didn't follow new rules

* fixed test so it uses new rule

* made exceptions to rule in interval_variable

* word fix

support >> supports 
says >> ways

* : for emphasis

* wip: design update for navbar with kiosk mode button

* feat: progress on new view mode button

* css: view state refactorings

* feat: kiosk modes & playlist support

* feature: cycle tv mode feature, renamed view modes to TV, and Kiosk

* fix: updated the alert notification message

* fix: removed unused parameter

* fix: correct the css class set for tv mode

* some minor improvements to playlist

- clear `edited` state for each query when a new datasource is set

* changed var to const

* fixed typo created in last commit

* added or empty object to options in prometheus/datasource