* improve remote image rendering

- determine "domain" during Init() so we are not re-parsing settings
  on every request
- if using http-mode via a rednererUrl, then use the AppUrl for the
  page that the renderer loads.  When in http-mode the renderer is likely
  running on another server so trying to use the localhost or even the
  specific IP:PORT grafana is listening on wont work.
- apply the request timeout via a context rather then directly on the http client.
- use a global http client so we can take advantage of connection re-use
- log and handle errors better.

* ensure imagesDir exists

* allow users to define callback_url for remote rendering

- allow users to define the url that a remote rendering service
  should use for connecting back to the grafana instance.
  By default the "root_url" is used.

* improve remote image rendering

- determine "domain" during Init() so we are not re-parsing settings
  on every request
- if using http-mode via a rednererUrl, then use the AppUrl for the
  page that the renderer loads.  When in http-mode the renderer is likely
  running on another server so trying to use the localhost or even the
  specific IP:PORT grafana is listening on wont work.
- apply the request timeout via a context rather then directly on the http client.
- use a global http client so we can take advantage of connection re-use
- log and handle errors better.

* ensure imagesDir exists

* allow users to define callback_url for remote rendering

- allow users to define the url that a remote rendering service
  should use for connecting back to the grafana instance.
  By default the "root_url" is used.

* rendering: fixed issue with renderKey where userId and orgId was in mixed up, added test for RenderCallbackUrl reading logic

[skip ci]

postgres: graphical query builder

[wip]added empty list cta to team list

added Bitcoin as a currency option

* 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

Merge branch 'postgres-query-builder' of git+ssh://github.com/svenklemm/grafana into postgres-query-builder