feat: set username behind flag

[mubbsharanwar]

In the spirit of making registration experience quick and easy (more details on the proposal page in the wiki, we want to be able to remove username from the registration form. This creates the need to remove it everywhere on the UX too. As part of this initiative, we will be removing username from the headers and making the dropdown consistent with edx.org site.

Changes Added another variation where header is rendered without the username. This variation is kept behind the HIDE_USERNAME_FROM_HEADER environment variable. By default, this feature is turned off.

Screenshots

Component	Feature flag disabled	Feature flag enabled
Header
Studio Header
Learning Header

Ticket VAN-1804

[mubbsharanwar]

@zainab-amir Addressed your comments.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3b2a2bf) 66.96% compared to head (fcaa2b6) 68.09%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #465 +/- ## ========================================== + Coverage 66.96% 68.09% +1.12% ========================================== Files 25 25 Lines 339 351 +12 Branches 78 85 +7 ========================================== + Hits 227 239 +12 Misses 110 110 Partials 2 2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[adamstankiewicz]

@mubbsharanwar Given this is making a change to the shared Open edX component header, should we seek reviewers/approval in the community Frontend Working Group or other stakeholders from the Open edX community?

Otherwise, if these changes are specific to a 2U/edX.org decision, should these changes be made in the frontend-component-header-edx repo instead? Generally, the Open edX version of the shared header component is replaced by @edx/frontend-component-header-edx via an NPM alias during build+deploy in 2U/edX's GoCD pipeline for MFEs deployed to *.edx.org.

[zainab-amir]

@adamstankiewicz thank you for sharing the information on the override for frontend-component-header. We will be making these changes to @edx/frontend-component-header-edx (we won't even need to keep this change behind a flag!)

[jmakowski1123]

Thanks for flagging this for product review. The linked "MVP scope doc" which I presume has additional product context to help make a decision is not available. Could this doc please be added to the Open edX product wiki? I created a space where you can copy/paste the doc: https://openedx.atlassian.net/wiki/spaces/OEPM/pages/4060938253/Proposal+-+Set+username+behind+flag+across+MFE+headers

[zainab-amir]

This PR is not needed anymore. For edX we have solved this by npm aliasing the header with edX specific header.