Allow to create custom pages and add them to the menu (with fixes from tests)

[apollo1220]

This PR is based on the PR by silenceway here
I had to add a markdown dependency to the requirements.txt and an id to an element for easier testing

From original PR: "This PR creates a new link in the admin page to go to a list of pages or allows to creation of new pages.

It saves basic page information in the database and it creates a markdown file in a pages directory within the config folder.

It shows pages in the sidebar before the books list or after the About link.

Pages can be enabled or disabled to hide them."

Will link PR for tests when created. Edit PR for tests here

[digitalfredy]

Hello folks, any news on this MR, I need this feature to add privacy notes 🙏

[apollo1220]

@OzzieIsaacs could you take a look at this and it's corresponding MR in the tests repo

[digitalfredy]

@OzzieIsaacs I thanks you for all the time you give to this project, from our side we are doing our best to contribute and to give to the project this feature we require instead of doing an internal hack in our own deploy.

• Sept 2023 the first PR was created here and it was reviewed and updated
• Oct 2023 Reviewer from the community give his OK
• Dec 2023 @apollo1220 from the community asked for this PR and you requested tests and he did it (thanks)

The organization I work for requires 2 or 3 additional features and I'm tell them that the right way with Open Source is doing contributions but to me will be hard to convince them if it takes 6 months to get the PR done. @OzzieIsaacs Please take a look 🙏 and again thanks for all your work.

[apollo1220]

Image containing this changed is published to apollo1220/calibre-web on Docker Hub