Update mentions of Riot to Element

[anoadragon453]

Description

Riot has rebranded to Element. At some point links to riot.im will probably stop working. This PR updates riot.im/app to instead point to app.element.io.

Status

READY

Type of change

• Documentation (fix or adds documentation)

How Has This Been Tested?

Checked the links, they seem to work.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #1743 (7179a4b) into master (101c317) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1743   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          71       71           
  Lines        4196     4196           
=======================================
  Hits         4172     4172           
  Misses         24       24           

Impacted Files	Coverage Δ
opsdroid/connector/matrix/html_cleaner.py	100.00% <ø> (ø)

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[FabioRosado]

Hello @anoadragon453 thank you for raising this PR and going through fixing all of these. I'm happy with the changes and to merge it 👍