Add a page loading indicator for the incident listing

[elfjes]

With custom page sizes (#145), incident list loading can take a while. In order to let the user know that the page is reloading, we should add an indicator.
This indicator should be visible when the incident list is reloading due to:

• a change in filters
• a change in page size
• a new page was requested

The indicator should not be visible when we're reloading due to auto-refresh

This issue is part of #46