Remove manual internal URLs from templates

There are currently ~150 manual URLs in the templates. These should be generated using Cake functions, to aid changes in routes as well as avoiding mistakes and the different styles of code used to make the URLs when they are not static. Exceptions include:

• (external URLs)
• other URLs outside of Cake (e.g. JS, CSS, images, but also commodity-viz)
• mailto: URLs
• anchors (#something, used to scroll to different parts of the page)
• maybe / because it would be overkill otherwise

Where possible, this should use <?= $this->Html->link(...) ?> directly instead of <a href="<?= $this->Url->build(...) ?>">.