Make corporate/individual signature for CLA more obvious

[rwinch]

When on the landing page it simply says thank you for signing the agreement which is confusing. It would be nice if we stated that there was a corporate CLA that covers the user and provided a hit (i.e. GitHub organization or email domain) as to why they are covered by the corporate agreement.

Relates to #130

[wilkinsona]

Perhaps both of the "Sign …" buttons could also be removed when the user's already signed or is already covered by a corporate CLA? Admittedly that would make have made it harder for me to do what I was trying to do which is to see what things would look like for a user that is trying to sign the CLA but that's very much an edge case.

[rwinch]

@wilkinsona Thanks for the suggestion.

The problem is that there are times when the user may want to sign as an individual in addition to the corporate signature.

In terms of hiding the corporate signature button, perhaps we can change the wording to say view signature as users may want to see which CLA they actually signed.