Always use GitHub usernames with @

[iccir]

This PR ensures that a @ is followed by a GitHub username for all Weekly Meeting Notes.

Closes #12

Changes:

• In 2018 and 2019, we used Dropbox for taking meeting notes. Username tags were @First L, with "L" being the first letter of the last name. This caused readability issues.
• Various @First tags were corrected.
• Various @github-username-with-typos were corrected.
• Hackmd usernames like "QWhXj01mSwmTjk5kN1H_qQ" were corrected.

To preserve privacy, I only made replacements in the following scenarios:

• The user is a member of Matplotlib Developers or officially associated with the project.
• The user gave us explicit permission (thanks @story645 !)
• The user's GitHub username appeared elsewhere in the meeting notes.

In the few cases where this wasn't the case, and in cases where @ was used incorrectly (like @attending: @tacaswell, … or @todo: Check on…), I dropped the @.