[Dashboard] Environment Variables Page

[gtsiolis]

Problem to solve

Currently, Environment Variables are mixed with other user settings like API Tokens, Notifications, etc.

This is part of #3300. 🍕

User Experience goal

The user should be able to create, read, update, and delete (CRUD) environments variables for their account but also specify a target project for each environment variable.

Proposal

In this first MVC (minimum viable change), we can do the following:

1. Introduce a dedicated page for creating, reading, updating, and deleting environment variables.
2. (Optional) Rename Environment Variables to Variables to make it friendlier to the user but also allow future features like supporting other types (see Support multiline environment variables #2058), etc.
3. Show a modal dialog to prompt the user for a variable key, value, and project scope.
4. Hide the variable value by default on the variables list. See Initially hide environment variable values in Settings page #1534.
5. Allow the user to read, edit, and delete a variable from the list in a modal.
6. Make the variable visible only within the modal.

Components involved:

1. Text Field
2. Button
3. Modal
4. Table or List

Future steps

1. In the future, we could introduce a button to toggle visibility of the variable value.
2. Suport multiline variables.

Designs

List	Empty	Create	Edit

For spacing guidelines, see similar patterns in #3399.

See also design specs.

Documentation

No documentation changes are needed for this first iteration.