The page path field allows invalid character sets. #1982

Describe the bug
The page path field allows leading and trailing slashes.

To Reproduce
Steps to reproduce the behavior:

1. Create a new page in wikijs.
2. Edit the page
3. in path add a slash at the beginning and click ok and save.
4. Try editing the page, You should get a 502 error from the browser.

Expected behavior
When clicking ok or save you should get an error informing the user that slash is not a valid first character.
Validation should also be added to block trailing slashes and double slashes. with appropriate error messages.

Host Info (please complete the following information):

• OS: [Ubuntu 20.04
• Wiki.js version: 2.3.77
• Database engine: [postgresql 12.2