Update check overlay

Starting with version 1.5.0 OctoPrint now supports the use of a centralized update check overlay, to allow for setting new software update configs for plugins that e.g. were abandoned by their original maintainer and have been adopted by a new one at a new repository location, or plugins that shipped with a broken default update config.

The overlay is hosted here on plugins.octoprint.org as a JSON file and currently contains the following entries:

Plugin Overlay
octoprint
{"user":"OctoPrint"}
Detailed Progress
{"user":"tpmullan"}
FilamentManager
{"user":"OllisGit"}
OctoKlipper
{"user":"AliceGrey"}
CostEstimation
{"user":"OllisGit"}
filamentreload
{"user":"ssorgatem"}
Marlin EEPROM Editor
{"user":"cp2004"}
Printer Statistics
{"user":"AlexVerrico","repo":"octoprint-stats"}
Continuous Print
{"user":"Zinc-OS"}
M73 ETA Override
{"user":"gdombiak"}
WebcamTab
{"user":"gruvin"}
OctoPrint-Mmu2filamentselect
{"user":"tkoecker"}
Slack
{"user":"mkevenaar"}
MetadataPreprocessor
{"user":"awenelo"}