Consistent use of relative URLs, and make BASE_URL optional

[jefft]

Jethro is full of href= and src= URL references, and is not too consistent in their use:

• Some are page-relative, href="?call=..."
• Some are incorrectly root-relative, href="/?view=...". These won't work if there is a URL path prefix e.g. /demo in https://easyjethro.com.au/demo
• Some are correctly root-relative, href="<?php echo BASE_URL; ?>/?view=..."

This patch gets things more consistent by:

• using page-relative URLs by default. Page-relative works fine, and doesn't have the code ugliness of <?php echo BASE_URL; ?> everywhere.
• Use the BASE_URL prefix where necessary, i.e. before /resources, /public and /members URLs.
• Remove the need for BASE_URL to end with a slash

By getting things consistent, it's actually possible to omit BASE_URL altogether, or set it to '' or '/demo'. This was my original goal: I want Jethro to run wherever it finds itself. If my Jethro normally runs at https://jethro.mychurch.com, by setting BASE_URL='' I can run it at http://localhost/mychurch without changing anything.