Enhance UX on Environment Form with Loading Icon

What does this MR do?

The loading icon adds nice feedback to the user that something is happening, but we only unset it in the error case as the good case (in both editing and creating) navigate us to a new page anyway.

Also disable the submit button if the form does not pass validations. The validations are pretty simple, so if they don't pass, there's no way the backend will accept it.

Finally, delete the HAML form. We don't need it anymore.

Changelog: changed

Screenshots or Screencasts (strongly suggested)

Screen_Recording_2021-07-23_at_10.56.51

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

For #336744 (closed)