Added support to send a Telegram notification & a few minor improvements

[ethans]

1. Added support to send notification to Telegram
2. Set the paths variable for the "Added Files Hook"
3. Fixed a bug in the hook that failed due to overriding the categories_verbose with the slug
4. Added in the notification hook a couple of environment variable checks before using them (so that it doesn't fail on forks)

During my testing I added a file to the notices folder and then deleted. That's why there are so many writes and deletes. Sorry.

Contributor Checklist

For Site Improvements

• [x] Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by the Bug Alert team if needed.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generate no new warnings.
• [ ] I have run Pelican locally to ensure my changes have not disrupted the look, feel, or functionality of the site. This worked for me yesterday, but post reboot - I'm unable to reach the site - I get that the connection is refused. But, I didn't make changes in the pelican site.
• [x] I have checked my code and corrected any misspellings.
• [x] I have added a pull request description which details the nature of this change.

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[sullivanmatt]

I'd feel better getting @mattlorimor's eyes on this too, before merging.