Custom Temps

[jneilliii]

• Your changes are not possible to do through a plugin and relevant
  to a large audience (ideally all users of OctoPrint)
• If your changes are large or otherwise disruptive: You have
  made sure your changes don't interfere with current development by
  talking it through with the maintainers, e.g. through a
  Brainstorming ticket
• Your PR targets OctoPrint's devel branch if it's a completely
  new feature, or maintenance if it's a bug fix or improvement of
  existing functionality for the current stable version (no PRs
  against master or anything else please)
• Your PR was opened from a custom branch on your repository
  (no PRs from your version of master, maintenance or devel please),
  e.g. dev/my_new_feature or fix/my_bugfix
• Your PR only contains relevant changes: no unrelated files,
  no dead code, ideally only one commit - rebase and squash your PR
  if necessary!
• Your changes follow the existing coding style
• If your changes include style sheets: You have modified the
  .less source files, not the .css files (those are generated with
  lessc)
• You have tested your changes (please state how!) - ideally you
  have added unit tests
• You have run the existing unit tests against your changes and
  nothing broke
• You have added yourself to the AUTHORS.md file :)

What does this PR do and why is it necessary?

Added modifications to allow for the injection of additional temperature data by plugins, which allows for graphing by a replacement Temperature tab.

How was it tested? How can it be tested by the reviewer?

Ran in development environment for some time both with default temperature tab and custom OctoPrint-PlotlyTempGraph tab that graphs added data injected by plugins, ie my test plugin here.

Any background context you want to provide?

What are the relevant tickets if any?

#2194, #1187, #321, #3261

Screenshots (if appropriate)

Further notes

Additional discussion on the forum here.