This commit adds a customizable timeout for screenshots called
capture_timeout. The default value is 10 seconds, and the maximum
value is 30 seconds. This timeout should be less than the minimum
Interval of all Evaluation Groups to avoid back pressure on alert
rule evaluation.
* Alerting: Change screenshots to use components
This commit changes screenshots to use a number of components instead of a set of functional wrappers.
It moves the uploading of screenshots from the screenshot package to the image package so we can re-use the same code for both uploading screenshots and server-side images; SingleFlight from the screenshot package to the image package so we can use it for both taking and uploading the screenshot, where as before it was used just for taking the screenshot; and it also removes the use of a cache because we know that screenshots can be taken at most once per tick of the scheduler.
