License 'nag' text fix to make consistent

License 'nag' text that an admin sees when a site has exceeded the licensed number of users needs to be made internally consistent. It refers to 'users' in two places and 'seats' in one. For example:

Your GitLab license currently covers 20 uers, but it looks like your site has grown to 46 users. Please contact sales(at)gitlab.com to increase the seats on your license. Thank you for choosing GitLab.

The "seats on your license" text above should be changed to "number of licensed users" The last sentence of the text should look like this:

Please contact sales(at)gitlab.com to increase the number of licensed users. Thank you for choosing GitLab.