Created resolve_to_valid_path() to return a valid zuliprc path.
Added docstring.

Previously, zuliprc_path was set directly in main().
Now, we are setting it 2 levels deeper, inside of login_and_save(),
passing it to main() through resolve_to_valid_path().

main() now sets the default value of zuliprc_path to None, and uses the
returned path from resolve_to_valid_path().

This is done to allow adding support for multiple zuliprc paths later,
where we will dynamically compute the path in login_and_save(), after
taking user input.

In addition to the previous default path ~/zuliprc.

Introduce the .config path of zulip-terminal as a str constant.

Add a function check_for_default_zuliprc() to search for zuliprc files
in both the default locations, when the --config-file argument is not
used.

It is added to ensure that users who have only a single account can
skip using the account alias argument that will be added.

Docstring updated.
Tests updated.

Newly generated zuliprc files were being saved to `~/zuliprc`.
This moves default zuliprc creation into the user's config directory.

The --config-file option will no longer be able to save zuliprc files
in custom locations.

The new config path including the .config directory are handled safely,
to allow creation even if they do not already exist.

Tests updated.

The test with id: `path_does_not_exist`, in the test function
`test_main_cannot_write_zuliprc_given_good_credentials()` has been
removed, as the path will now be created even if it does not exist.

The usage of the account-alias positional argument is made mutually
exclusive with the config-file option.

Lists all the accounts in the config path, by checking the directory
structure. The home path zuliprc is not considered in this list.

Tests updated.

Previously, we had to pass the zuliprc_path because the --config-file
option could be used to create a zuliprc at a custom location.

Now, all accounts are only saved to the standard config path. So, by
moving the print statement to the outer function, we can simplify
login_and_save().

- If --new-account is used along with --config-file,
  - If the given path is valid, that zuliprc will be loaded
  - Else, a new account will be created, but not at the given path.
- If --new-account is used along with account-alias, the new account
argument will be ignored.

Tests updated.