Optional url splitter

[MMikePL]

Summary
This PR allows disabling URL splitting during submission by leaving the url_splitter configuration value empty and handling such value in the code. It also changes the default behavior to not split URLs, addressing issues where valid URLs containing commas (the previous default) were incorrectly fragmented.

Argument for Change
The previous default splitter was a comma (","). However, commas are valid characters in URLs (e.g., in query parameters or certain path structures). Using a comma as a mandatory default splitter caused many legitimate URLs to be incorrectly broken into invalid fragments during submission. This change ensures that by default, URLs are preserved exactly as entered, while still allowing users to opt-in to multi-URL submission by configuring a specific delimiter.

Configuration Limitation
Due to how configuration values are parsed and stripped, setting url_splitter to a space is effectively treated as an empty value (disabling splitting). If space-delimited splitting is required in the future, a more complex solution may be necessary, such as implementing a specific placeholder like "whitespace" in the config that the code would then interpret as a command to split by whitespace.

[gemini-code-assist]

Code Review

This pull request updates the URL splitting logic in the submission view to allow disabling the splitter by leaving the configuration empty. I have reviewed the changes and suggest incorporating a list comprehension to strip whitespace and filter out empty entries during the split process to improve robustness against malformed input.