chore(plugins): Add new plugins

[merkata]

Overview

Adds three new plugins requested by Community team:

• https://github.com/discourse/discourse-templates
• https://github.com/discourse/discourse-calendar
• https://github.com/discourse/discourse-gamification

Rationale

Requested.

Juju Events Changes

n/a

Module Changes

n/a

Library Changes

n/a

Checklist

• [x ] The charm style guide was applied
• [x ] The contributing guide was applied
• [x ] The changes are compliant with ISD054 - Manging Charm Complexity
• [x ] The documentation is generated using src-docs
• [x ] The documentation for charmhub is updated.
• [x ] The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions]

Test coverage for 5be0b56

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        325     31     82     11    90%   137, 171, 179-180, 336->341, 533-535, 540-541, 552-554, 559-560, 572-574, 579-580, 592-594, 619-621, 661->exit, 671->exit, 701-707, 733->exit, 747-748, 758->exit, 772
src/database.py      30      1      8      1    95%   56
-------------------------------------------------------------
TOTAL               355     32     90     12    90%

Static code analysis report

Run started:2023-12-12 08:38:59.409087

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1944
  Total lines skipped (#nosec): 1
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):