Entry management: Make Link validation regex more permissive

[rlrh]

This is the current validation regex for Link:

private static final String PATH = "[-a-zA-Z0-9+&@#/%?=~_|!:,.;]*[-a-zA-Z0-9+&@#/%=~_|]";
public static final String VALIDATION_REGEX = "^(https?|ftp|file)://" + PATH;

We can modify it to be more permissive as file: does not need double slashes.

With this change, entries in TypicalEntries can be simplified from
"file://" + MainApp.class.getResource("somefile").toExternalForm().substring(5)
to just
MainApp.class.getResource("somefile").toExternalForm().

[epicfailname]

public static final String VALIDATION_REGEX = "^(https?|ftp|file)://" + PATH;
to
public static final String VALIDATION_REGEX = "^(https?://|file:/(//)?)" + PATH;

[thomastanck]

lgtm