dhcp.cgi: Fix bug#10629 - Highlight fixed IP's in dynamic range in red

dhcp.cgi: Fix bug#10629 - Highlight fixed IP's in dynamic range in red

- This v2 patch has moved the extraHead variable into header.pl
- This patch marks all IP's that are in the Fixed list but are also in the dynamic range
   that has been defined, in red.
- Additional function created to check if an ip address is in a defined range.
- Added an additional key item under the Fixed Leases table for Fixed IP in dynamic range
- Added line to English Language file for this key item.
- ./make lang run before commit.
- Tested in vm testbed and confirmed that any ip address in the Fixed Leases table that
   is in the defined dynamic range is highlighted in red
- This uses the css background-color appoach from the first patch in this set.
- This patch only highlights those IP's that overlap in red but does nothing more. So a
   user can still create new ones if they want but they will all show up in red.
- This patch flags up if people are doing things that they shouldn't be doing but allows
   them to continue doing so without changing anything if they don't want to and so will
   not break existing setups.

Fixes: Bug#10629
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>
Tested-by: Bernhard Bitsch <bbitsch@ipfire.org>