Disable apply button when there's a pending application

Purpose

The purpose of this PR is to fix #132

The apply button should be disabled when there's a pending application with a tooltip mentioning you can apply only for one mentor at a time

[x] This PR doesn't commit any keys, passwords, tokens, usernames, or other secrets.
[x] I have read and understood the development best practices guidelines ( http://bit.ly/sef-best-practices )
[x] My code follows the style guidelines of this project
[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] I have made corresponding changes to the documentation

## Test environment ## Learning