Migrate signup page to angular router.

[ashutoshc8101]

Overview

1. This PR fixes or fixes part of NA
2. This PR does the following: Migrate signup page to angular router.

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Proof that changes are correct

signup-page-2021-08-05_14.49.35.mp4

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[oppiabot]

Hi @ashutoshc8101, can you complete the following:

1. The proof that changes are correct has not been provided, please make sure to upload a image/video showing that the changes are correct. Or include a sentence saying "No proof of changes needed because" and the reason why proof of changes cannot be provided.
   Thanks!

[oppiabot]

Hi, @ashutoshc8101, this pull request does not have a "CHANGELOG: ..." label as mentioned in the PR pointers. Assigning @ashutoshc8101 to add the required label. PRs without this label will not be merged. If you are unsure of which label to add, please ask the reviewers for guidance. Thanks!

[vojtechjelinek]

LGTM!

[oppiabot]

Unassigning @vojtechjelinek since they have already approved the PR.

[oppiabot]

Unassigning @nithusha21 since they have already approved the PR.

[oppiabot]

Hi @ashutoshc8101, there is a new change in develop which needs to be in your PR. Please update your branch with the latest changes in develop. For instructions, refer to this link. Thanks!

[oppiabot]

Hi @ashutoshc8101. Due to recent changes in the "develop" branch, this PR now has a merge conflict. Please follow this link if you need help resolving the conflict, so that the PR can be merged. Thanks!

[DubeySandeep]

LGTM for the codeowner file main.py left or comment, PTAL!

[DubeySandeep]

Can you explain what's MANUALLY_REGISTERED_WITH_BACKEND and why it is introduced? :)

[ashutoshc8101]

Some URLs contain URL fragments and they have different syntax for angular router and backend.
eg.
Url for backend: /learn/<classroom_url_fragment>
url for angular router: /learn/:classroom_url_fragment

So, I have used the second case in constant.ts (so, it works directly with the angular router).

And manually registered these routes with fragments in the backend.
That's why I am excluding these routes in this loop.

This explanation is for other pages with URL fragments in PR #13533.

For, /signup route, it does not use the common Handler OppiaRootPage unlike others. It uses SignupPageHandler as it performs additional checks too.

[ashutoshc8101]

@DubeySandeep PTAL

[oppiabot]

Unassigning @darksun27 since they have already approved the PR.

[oppiabot]

Hi @ashutoshc8101, the build of this PR is stale and this could result in tests failing in develop. Please update this pull request with the latest changes from develop. Thanks!

[oppiabot]

Unassigning @DubeySandeep since they have already approved the PR.