Add default page title

[mpourismaiel]

What this PR does / why we need it: Add default page title using title of the first content fragment found in page.

Which issue this PR fixes: fixes (partially) #655

Special notes for your reviewer: This PR only adds default title to the page. Removing date metadata which was mentioned in the issue is not done and I don't think fixing it in the same PR is a good idea since the change is a bit complicated already.

Release note:

- page: Each page now uses the first content fragment's title as page title by default.

[ghost]

DeepCode's analysis on #1fe8e6 found:

• :x: 0 critical issues. :warning: 0 warnings and 0 minor issues. :heavy_check_mark: 0 issues were fixed.

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