Add sanitize function for redirect parameter next

[VladyslavTy]

Change description

Problem: The next parameter used with the /login route is incorrectly decoded, so the redirection happens to the wrong URL. When the next parameter contains a + sign, the application encodes it as a space value. For example:
...?next=courses/course-v1:snowflake-sandbox+ACC-PTF+C/course - at the moment of redirection this route will be encoded as ...?next=courses/course-v1:snowflake-sandbox%20ACC-PTF%20C/course where the %20 is encoded space symbol. So, the user will see the 404 page instead of the course page.

Fix:
Add the sanitize function which will check the next parameter:

• If the next parameter looks like the course route - replace the + symbols with the right ASCII code %2B
• If the next parameter is not the course route - keep this parameter as it is

Type of change

• Bug fix (fixes an issue)
• New feature (adds functionality)

Related issues

Related to ENG-567

Checklists

Development

• Lint rules pass locally
• Application changes have been tested thoroughly
• Automated tests covering modified code pass

Security

• Security impact of change has been considered
• Code follows company security practices and guidelines

Code review

• Pull request has a descriptive title and context useful to a reviewer. Screenshots or screencasts are attached as necessary
• "Ready for review" label attached and reviewers assigned
• Changes have been reviewed by at least one other contributor
• Pull request linked to task tracker where applicable

[jfaMan]

LGTM 👍 Appreciate the helpful description!

One of the checks mentions a merge conflict though.

[github-actions]

Checking git merge conflicts against https://github.com/openedx/edx-platform.git

Comparing with	open-release/nutmeg.master
Benchmark conflicts with main	299
Current conflicts	301
Summary	Adds 2 new conflicts. How can we do better?

Comparing with	master
Benchmark conflicts with main	329
Current conflicts	331
Summary	Adds 2 new conflicts. How can we do better?