Add icon pattern for Identifiers screen

[jspaaks]

blocked by decision in #455

[jspaaks]

To summarize the discussion from #477 (comment),

• Add question texts above each input; include SchemaGuideLink icons
• Add more space between the user input elements
• Let the question text of identifiers[].value update with the value of the radio button
• Let the question text of identifiers[].description update with the value of the radio button
• Don't work on wrapping the error messages yet.

Below are some visual mockups:

When radio button is set to DOI:

When radio button is set to URL:

When radio button is set to SWH:

When radio button is set to Other:

[jspaaks]

PR #495 was merged, closing this issue