Bugfix: footer in edit details

[ErwinKomen]

When in edit details, the Save button is not visible enough anymore, due to the footer

[ErwinKomen]

Observations

• The actual save button is in save.html
• It is included in all add_event.html and add_(anything).html files.
• The place where it is included is inside the <form> element, which is inside the block content

As for the footer:

• that is inside base.html
• the block content is inside a <div> which is above/before it

[ErwinKomen]

Actions

1. Adapt site.css and added:
   1. rule for .container-fluid > .row > .col-10 to make sure the content material is above the save buttons
   2. rule for a div of class main-container with a margin-top value specified, in order to be below the fixed-top navbar
2. Adapted base.html:
   1. Added an over-arching div with class main-container (see above for the css)
3. Adapted navbar.html:
   1. Add class fixed-top to the <nav> element to create a fixed top menu