Use text field to capture further details of Other role type during signup
### Problem
Currently, our second most popular selection at signup for the `Role` question is `Other` which leaves us in the dark about what type of user is signing up for GitLab ([source](https://app.periscopedata.com/app/gitlab/551863/Growth-Conversion-Team:-SamA-Scratch-Dashboard?widget=9422831&udv=949690)).
| User experience | Supporting data |
| ------ | ------ |
|  |  |
### Solution
For one week, we implement a conditional open text field whereby if a signup selects a value of `Other` in the `Role` question we display a free text field for them to provide us with their role. This field should be optional, we should only run this test for a week or two as this will hurt the completion rate of the form since we're introducing another question in the form. However, we can utilize the results to understand if there are more appropriate options we should be including in the role question moving forward. We will turn the experiment off once we feel like we have a sufficient number of responses to update the picklist.
This data should be stored on the backend in the equivalent area where we're already storing the role data as an additional field titled `role-other`.
### Proposed solution

#### [Design specs](https://www.figma.com/file/7x9B11kesCFIQzKiYe9ATo/Contextual-input-field-for-roles-in-registration-process?node-id=15%3A0)
epic