[Frontend] Login functionality improvements and unit tests

[berkaydoner]

I had not implemented a input validation method over the email and password fields in the forgot password page before. I am going to add them with two helper methods to check email and password formats. I am also planning to try various strings and check if they are in email/password format in the unit tests.

[berkaydoner]

I have added two methods to check the email and password format.
Email format check method checks if there is a segment separation in the text specified by @ and if there are one or two segment separations after @, specified by dot. Texts with "...@domain.dom" and "...@domain.dom.do" format are accepted as emails.
Password format check method checks if the given string contains lowercase, uppercase characters and digits and has a length of at least 8.

[berkaydoner]

I have added unit tests with various possible email and password texts and check if they are correctly identified or not. My unit tests pass on my local system and in the GitHub Actions. Pull request is #304.

[berkaydoner]

Pull request is merged and closed.