Miscellaneous config issues fixed

[rg663]

Resolves #414 —

Now, the following occur:

• the Help button on the navbar only appears when the user specifies the center's help URL, otherwise is not present
• when removing a user from an allocation or a project, the alert reads "removed 1 user" instead of "removed 1 users"
• the default center name is HPC Center to avoid redundancies in parts of the website

Tested through:

• specifying a center help URL and seeing the button appear, and not specifying one and seeing the navbar without the button
• removing a user from both a project and an allocation and reading the alert
• checking the default center name in config settings and in parts of the frontend, such as when requesting an allocation