fixes OpenTreeMap/otm2-addons#368
Removed comments
Remove camel case from html
Break status into inheritance pattern
Make GenericImportEvent.errors.data always a []

Forcing this to be a list greatly simplifies template rendering of these
errors. We can say if errors.data then make a <ul> of each member of
data. For row-level validation this is not so simple, because each error
type appears to have its own schema, but for small number of cases of
file-level errors, it's easy to force this structure.
Make errors use client-appropriate summary text

In otm-legacy, these errors had their own internal status messages which
were more or less duplicated in user-friendly language in the
templates. Since they're plain text and not computer-friendly anyway, we
might as well just store the client friendly text and render as-is.
Show error summary page for file level errors
Prefer generalized errors over specific cases

The generalized MISSING_FIELD error accepts strings of text that
represent the actual fields that were missing. Adding a MISSING_FIELD
error with ['point_x', 'point_y'] or one for each offers the same level
of detail as MISSING_POINTS without requiring the existence of an extra
type. The same goes for MISSING_SPECIES_FIELD. These error types did not
add any additional detail in their error message anyway.
Validate even when there are no rows

We have an error case for EMPTY_FILE, but it was impossible to reach this
case because if the reader object did not iterate once, the file level
validations were not executed.
Merge pull request #1797 from steventlamb/topic/importer_file_failure_view

Topic/importer file failure view
Rename importer's plot ID fields

`OPENTREEMAP_ID_NUMBER` -> `OPENTREEMAP_PLOT_ID`
* Make it clear this is the Plot ID, not the Tree ID
* `PLOT` instead of `PLANTING_SITE` following existing fields like `PLOT_WIDTH` and `PLOT_HEIGHT`

`ORIG_ID_NUMBER` -> `EXTERNAL_ID_NUMBER`
* `EXTERNAL` clearer than `ORIG`
* Retain `NUMBER` since this is a user-populated field, to clarify what we expect

Note this changes no model attribute names, so effects are localized to the importer.
Create separate functions for committing plot data and tree data
Importer saves UDF values for trees
Fix importer documentation
Merge pull request #1799 from steventlamb/bug/fix_importer_error_documentation

Fix importer documentation
Add `transaction.atomic` to `commit_rows`
Merge pull request #1798 from RickMohr/save-udf-values

Importer saves user-defined field values for plots and trees
When committing tree imports, update plot if it is already present
Get importer commit working, and refactor it to return a partial
Remove irrelevant commented out HTML