Fix miscellaneous config issues

[dsajdak]

In thorough testing of the UI after the bootstrap 4.6 upgrade, @rg633 found a few minor, but important, items to fix. Detailing them here as they come up:

• make it so the help button only shows up when the CENTER_HELP_URL is specified
• When removing users from an allocation or project, if only 1 user is removed, it says “removed 1 users” - change that to “removed 1 user”
• Change the default center name to 'HPC Center' instead of 'HPC Resources' - When requesting an allocation the center name gets joined with 'resources' so it becomes “HPC Resources resources”