[HOLD]#159687667 Enable manager to delete users and trainers from a gym

[ja-odur]

What does this PR do?

• Enable gym managers to delete users' and trainers' account from the gym, not the application.
• Enable gym administrators to add already existing users to the gym.

Description of Task to be completed?

• Modify UI to enable gym managers adding already existing users to the gym.
• Add GymAddExistingUserForm form and GymAddExistingUserView view.

How should this be manually tested?

• Clone this branch and follow the instructions stated in README to set up the application.
• After setting up the application, start the server by running the command python manage.py runserver in the terminal with your virtual environment set.
• In the browser, log in as an Admin and go to the gyms page. As an admin or a gym manager, you should be able to delete, and create users as shown in the screenshots section.

What are the relevant pivotal tracker stories?

159687667

Screenshots

Screenshot before the implementation

Screenshots after implementation

[coveralls]

Pull Request Test Coverage Report for Build 369

• 75 of 85 (88.24%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 93.191%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wger/gym/forms.py	14	17	82.35%
wger/gym/views/gym.py	36	43	83.72%

Totals
Change from base Build 342:	0.2%
Covered Lines:	12387
Relevant Lines:	13292

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💛 - Coveralls