Fix broken links and update Slack logo.

[sam-thibault]

This change consolidates a number of fixes for broken links (Slack community, readme, links containing "master"), and also updates the Slack logo.

Related issues and proposed fixes include:

• https://github.com/moby/mobywebsite/issues/54

• https://github.com/moby/mobywebsite/issues/55

• https://github.com/moby/mobywebsite/pull/49

• https://github.com/moby/mobywebsite/pull/59

• fixes https://github.com/moby/moby/issues/39170