Code cleanup when adding user tags.

Use Object.saveAll instead of multiple save requests joined by a promise. The
old way could fail because the burst of individual requests could be hitting
the burst limit to the server.

Also use 'increment' to do atomic incrementation of number of tags on the
server.
Improve error handling in login/singnup form.

Disable the buttons when the form is invalid.
Provide localized error messages.