Connection timeout + check API integration

[matthewhilton]

TLDR

• Adds connection timeout Closes Add timeout for requests #46
• Cleans up status check Closes The status check should only occur on the settings page itself #107

Changes

• Configurable Guzzle http timeout set to default 5 seconds.
• Refactored the engine status check to provide better error messages (e.g. URL gave http code XYZ)
• Implemented check API https://moodledev.io/docs/apis/subsystems/check
• Created admin setting which takes in a check class and displays it nicely. (Note this will eventually be integrated a bit nicer likely with ajax in core https://tracker.moodle.org/browse/MDL-67898)
• This check now only runs when you are viewing the full tree (i.e. not searching) and when you are on the elasticsearch config page. Since it can take up to the timeout (5 seconds default) - we don't want this running when viewing other pages, etc...

Screenshots

See #108 (comment)