Support generating sitemap for SEO

[lawrenceching]

WIP.
Good to have a early review.

[BPowell76]

Any chance on getting this merged? I'd like to add a sitemap to my wiki as I'm using it for my public-facing website. For the time being I can generate a sitemap by hand, but the more pages I have that's going to be tedious and annoying.

[BPowell76]

@lawrenceching Some things we should add to keep the sitemap compliant, is have an option to set the priority page in the sitemap. Valid values are 0.0 to 1.0. Another thing to add is a change frequency, but a default value could be assigned for monthly or weekly.

Adding the <priority> component to the sitemap and page properties editor will require a change to the database schema by adding a new column.

https://www.sitemaps.org/protocol.html

That said, Google Search Console is indexing my wiki without the sitemap being supplied to it. I guess providing the Google Analytics token in the Administration dashboard is good enough?

[lawrenceching]

@BPowell76
I used a default priority and change frequency for every page.
And allow users to change them in config file.

@NGPixel
I think my change is ready for review. Could you have a look, especially at the permission checking part?