Import Module improvements

[sleidig]

Based on initial user observations, there are a number of small-ish things to improve about our import UI:

• not intuitive what are columns, what are profile entity fields (visualization?)
• no warning indicator for date after selecting field, but format config is required
• if column headers are not in first row -> column mapping screen very confusing (!)
• dropdown options that are long and have a line break are hard to distinguish from the next option in the list
• imported data might contain multiple different date formats (with/without leading zero of month, ...)
• "continue" button sometimes overlooked
• empty strings are imported as "null"
• unmapped or empty values of enum props are shown as "[invalid option] undefined" in the preview (should just be empty)
• column mapping dropdowns are rather narrow, hard to read to full field names (--> give more width)
• save column mapping already when first continuing from the mapping step (in case the import fails the user otherwise has to re-select all the mappings ...)

value mapping popup:

• popup warning for date value mapping about "unmapped values" would better be phrased as "unparseable values" or the like
• warning popup for boolean value mapping "unmapped values" is misleading if the user actually wants to map some values to be - imported as "unchecked" (i.e. false) but there is no way to distinguish unmapped and mapped to "false" currently
• boolean value mapping: more visual clarity that left value will turn into right value (checkbox is not selecting what value from left is imported but represents the actual value after import)

preview screen:

• Import: need loading bar for preview #2429
• sort order of rows to match the imported file? or maybe an extra column giving the row in imported file?
• scrollbar horizontal not available in preview (some columns seem to be "missing" because they are out of the screen)