Add messages.html

[JustRomanVolkov]

and refactoring base.html, user.html

[github-actions]

Overall, the code looks good and doesn't have any major issues. However, here are a few suggestions for improvement:

1. Avoid Inline Styles: Instead of using inline styles to control the visibility of the message count badge, consider adding a CSS class for the hidden state and toggle it dynamically using JavaScript.

2. Use textContent Instead of innerText: When setting the text of the message count element, it's better to use textContent instead of innerText for security reasons.

3. Handle Error Cases in initialize_notifications(): Currently, if there is an error while fetching notifications or parsing the response, there is no error handling. Consider adding error handling to handle such cases gracefully.

4. Use Relative URLs for Pagination Links: In the messages.html template, the pagination links are currently empty. Update the href attributes to include the appropriate URLs or route names for navigating to newer and older messages.

5. Avoid Using _ as a Variable Name: In the messages.html template, you are using _ as the loop variable name for iterating over the messages. _ conventionally represents a variable that is not used, so using it as a loop variable might be confusing. Consider using a more meaningful name for the loop variable.

6. Use {% trans %} Instead of {{ _('') }}: Instead of using {{ _('') }} for translation, consider using the {% trans %} template tag provided by Flask-Babel. It provides better support for translation and avoids unnecessary template variable creation.

7. Separate Template Blocks: In the user.html template, consider separating the parts that deal with the current user and other users into separate template blocks. This can improve maintainability and make the template more modular.

Overall, the code is well-written and follows best practices. These suggestions should further improve the code's readability, maintainability, and performance.