[GUI][BUG] Properly validate proposal creation Name/URL

[Fuzzbawls]

Remove the hard input cap of the name/URL inputs in the proposal creation UI so that the underlaying validation methods can give visual feedback when input lengths are too long (prevents silent cut-offs when pasting a too-long string).

Sanitize both fields to ensure that no illegal characters are accepted, matching the RPC interface's functionality.

introduce a future-forward (not yet implemented) method of verifying name/url fields on the network level.

[DeanSparrow]

ACK dc11219

Compiled and tested using -regtest
Adding special characters to proposal name or URL changed outline of box to RED and error message appeared.
No issue adding proper name/URL.

Name is limited to 20 char
URL is limited to 64 char

[Fuzzbawls]

Compiled and tested using -regtest Adding special characters to proposal name or URL changed outline of box to RED and error message appeared. No issue adding proper name/URL.

Name is limited to 20 char URL is limited to 64 char

all as intended. the red outline indicates an issue, and trying to continue via the button will display the actual error message in the UI and will not allow further progress until the issue(s) are fixed.

[yenachar]

utACK dc11219

Reviewed the code. The overall approach to validation looks good, the new url-matching regular expression looks better, and string passing by const reference adds clarity.