Improve styling of Rename modal and modal cancel buttons.

[asolove]

Resolves #350.

Before	After

• Rename modal's submit button now says Rename
• All modal cancel buttons have blue, rather than gray, text, to show they are clickable and to match Google styles.

In the initial issue, @shriram asked:

I also feel like the very light grey of Cancel makes it looks like it's not enabled. Is the grey instead of red for cancel a consistent style?

I agree that the light gray only makes it look not enabled. This PR makes the text blue to match the styles commonly used in the Google/Chromebook apps that students will be familiar with. e.g. here is a sample from Google Docs:

In general, I'd recommend we not use red for non-destructive secondary actions like "close", as it has a connotation of something dangerous, whereas in this case close should clearly be taken as the neutral action with no side-effects. CPO no longer uses red for any secondary actions that I can find from clicking around.

[shriram]

Oh yeah, I really like this improvement! And matching GDocs seems to have value on Chromebooks and elsewhere. Thanks!

[jpolitz]

Looks good thanks!