Run schema validation during `qhub init`

We are currently only running schema validation when qhub render is called. Let’s move this sort of validation as early in the process as we can.

Here is any example of the qhub init passing but qhub render failing a validation check that should be caught earlier in the process, as part of this failed gh action:

$ qhub init "do" --project "test--do-github-actions" --domain "do.qhub.dev" --auth-provider github --disable-prompt --ci-provider github-actions
$ qhub render -c "qhub-config.yaml" -o "qhub-do-github-actions-deployment"

Error: The schema validation of the qhub-config.yaml failed. The following error message may be helpful in determining what went wrong:

1 validation error for Main
project_name
  
        In order to successfully deploy QHub, there are some project naming conventions which need
        to be followed. First, ensure your name is compatible with the specific one for
        your chosen Cloud provider. In addition, the QHub project name should also obey the following
        format requirements:
        - Letters from A to Z (upper and lower case) and numbers;
        - Maximum accepted length of the name string is [16](https://github.com/Quansight/qhub/runs/6178700354?check_suite_focus=true#step:10:16) characters.
        - If using AWS: names should not start with the string "aws";
        - If using Azure: names should not contain "-".
        
Maximum accepted length of the project name string is 16 characters. (type=value_error)
Error: Process completed with exit code 1.

Related to #761

[trallard]

@iameskild @asmijafar20 is this being covered in your current PR?

[iameskild]

In this PR, we are currently validating most of the inputs that are being passed in when running nebari init ... but there are a few more checks that should be added. Unfortunately most of the remaining checks are scattered throughout qhub/initialize.py and need to be moved to qhub/schema.py.