Add link to latest Summit videos

[spier]

Functional changes:

• Add links to Summit video recordings to sidebar, and also to list of events. (check only https://github.com/InnerSourceCommons/innersourcecommons.org/commit/a7184eabbfa6588fc01dbfb728723da1dbf938cf to see those)
• fix parsing error in latest board notes

Non-functional changes:

• changed links on events page to use markdown syntax rather than HTML
• fix code indentation of the HTML in the sidebar
• add Gemfile.lock to ignored files

[spier]

This PR will also fix the formatting issue on the last board report board/2020-11-18-terse-notes.md that you see here: http://innersourcecommons.org/board/2020-11-18-terse-notes/