changed SignUpActivity and Strings

[Ankit-akumar]

Description

A complete discussion over this can be found here.
Current Behaviour: Simply shows a toast to the user saying "Invalid XYZ".
Changed Behaviour: Gives a detailed error message for any invalid input.
Following screenshots would be helpful ->

Fixes #1137

Changed the validateDetails method of SignUpActivity.kt for setting the appropriate error message to be displayed to the user. Added the messages in strings.xml.
(And rearranged some lines of code unintentionally :-) )

Type of Change:

• Code
• User Interface

Code/Quality Assurance Only

• New feature (non-breaking change which adds functionality pre-approved by mentors)

How Has This Been Tested?

Steps to reproduce ->

1. Start the app.
2. Navigate to Signup activity by clicking the SignUp button.
3. Enter the following strings as test cases or create your own.

• name: B, username: bro, email: bro@bro.com, password: Bro@123. (use to check the required length of strings)
• name: B$, username: Big bro, email: bro@bro.com, password: BigBro@123. (use to check the required format of strings.)
• name: Big Bro, username: Big_bro, email: bro@bro.com, password: BigBro@123. (These are the valid inputs)

Checklist:

• My PR follows the style guidelines of this project
• I have performed a self-review of my own code or materials

Code/Quality Assurance Only

• My changes generate no new warnings

[epicadk]

Looks good. Thankyou. Needs @isabelcosta 's approval to run workflows.

[epicadk]

@isabelcosta can you please approve request to run the workflow?

[isabelcosta]

Thank you for contributing @Ankit-akumar ! Awesome work!

[isabelcosta]

@Ankit-akumar I noticed in your PR description, the way you linked the issue was not a way that would make GitHub close the issue once this one got merged. So I changed from:

**Fixes** # [[ISSUE](https://github.com/anitab-org/mentorship-android/issues/1137)]

to

Fixes #1137

and this way, github will know which issue on the platform you are referring to :)