Update to front page minigraphs and micrographs

Patch makes minor changes to the show_minigraphs() and show_micrographs() functions in html/pages/front/default.php.

1 - Skips empty graphs caused by a trailing or otherwise excess semicolon in the config.php ids definition.

2 - Custom minigraph dimensions always apply, device graphs did not use them.

3 - Fixes broken multi-graph links by urldecoding the id list if $type contains multi.

The ids list for mini and micro graphs uses a comma and semicolon delimited string, so comma-separated ids for a multi-graph must be provided in urlencoded form (%2C). This already works fine with the graph image src and they appear on the front page, but the link href undergoes a urlencode, taking you to a broken graph url that only renders the first id.