Add relative linking to menu item template and remove base url from header template

[jsigee87]

This PR fixes a relative URL issue that causes the following behavior:

• When deploying to a test location, the base URL of the test location is overwritten by the baseURL that is loaded into the <base> tag. This is solved by removing the line of HTML that generates that tag in the header.html template.
• When deploying in any environment, footnotes do not work. The generated footnote links are relative and so are appended to the base URL and the user is redirected to e.g. https://mybaseurl.com/#fn:1. Removing the base URL fixes this issue as well.
• When deploying to a test location, after clicking a menu item on the main page you are brought to a list of posts. Clicking on a post redirects the user to e.g. https://www.mybaseurl.com/menuitem/post instead of test-location/menuitem/post. This is fixed by changing the line in li.html to have .RelPermalink instead of .Permalink.

[jsigee87]

Thanks!