Implemented UptimeRobot widget

[franga2000]

Hi! This is the PR corresponding to my proposal in #979.

There are still a couple of things I'd like to implement, but I'm submitting the PR now to track my progress and see what CI has to say about my code.

Here's what it looks like now with my config:

uptimerobot:
      apiKey: --redacted--
      enabled: true
      uptimePeriods: 7-30

TODO

• Make symbols configurable
• Make colors configurable (maybe just a general prefix?)
• Improve the default formatting of uptime ratios
• Add option to sort monitors by status (pull offline ones to the top)

[senorprogrammer]

Looks good so far.

[senorprogrammer]

Just checking in: do you want this merged, or are you waiting on doing the other todo's?

[franga2000]

Sorry for the silence, I got a bunch of work thrown at me out of nowhere. It's probably best you merge it and if anyone asks for the customization options or I get some spare time in the future, I can send another PR.

[senorprogrammer]

This branch has been accepted and merged. Nice work!