Revamp design of applications and submissions tabs within profile

[julianweng]

Currently, open applications (specifically, Wharton Council applications) and a user's application submissions are accessible via the /apply route, which as mentioned in #773 is non-ideal as the apply page is somewhat hidden and inaccessible from the main page, and it separates submissions from requests (accessible in the profile page) which causes confusion.

#718 solves some of these issues by moving open applications and submissions to tabs within /profile. However, this is an imperfect solution: "submissions" as a label is unclear without the context of applications and this approach creates a lot of tabs (3) within profile. In addition, only returning open applications of Wharton Council-affiliated clubs is obviously non-ideal and makes the tab useless for the majority of the year.

It would be great to build off of this progress and design/implement a more elegant and intuitive hierarchy for displaying application related information within /profile. For instance, we can have one "applications" tab and three nested / second-order tabs beneath the existing tabs for open applications (which should be modified to return all open applications), submissions, and membership requests, but this is up to discussion.