Don't automatically add CNCF as an employer based on LinkedIn

A large number of LFX mentorship students add the CNCF to their LinkedIn profile. This means their contribution shows up in devstats as being related to the CNCF, which isn't really true.

Likewise, I joined the Governing Board recently and updated LinkedIn to reflect this. My affiliation was (automatically?) updated to CNCF in [this commit](https://github.com/cncf/gitdm/commit/f741231f3015eaf3764b0623590e5213f24844c6), and hand-set dates were lost in favour of automatically added ones

Some possible suggestions:

(a) don't add a new affiliation based on LinkedIn if the old one has not ended
(b) maintain a list of affiliations not to automatically add, and have the CNCF be on that
(c) audit everyone listed as "CNCF" to only actually be paid CNCF staff (optional)