Check URI validity

[lfarid]

If the users chooses to use the contents of a specific column as the subject URI, the URI needs to be checked for validity and consistency

[almey]

done.

[lfarid]

• if a user selects a column as the subject URI - this has to also be validated (see #140)
• Error message for the above should state: Please make sure the URIs are valid in the file.
• General error message should say : Please enter a valid URI (in English)

[bbrand84]

@almey can you have a look at it again? i removed the pattern and onvalid properties of the urri input because it wouldnt let me go to the next page, though I had a valid uri. don't know why. also, the red text "Please make sure the URIs are valid in the file." appears very ramdomly.