Return false if instanceuser is None

fixes

"""
ERROR: test_plot_detail (treemap.tests.urls.TreemapUrlTests)

[localhost] out:   File "/usr/local/otm/app/opentreemap/treemap/templatetags/instance_config.py", line 50, in is_deletable
[localhost] out:     return obj.user_can_delete(instanceuser.user)
[localhost] out: AttributeError: 'NoneType' object has no attribute 'user'
"""
Fix localization of tree ids

This was causing malformed urls on servers with >1000 trees, due to the presence of commas.
Use values() instead of objects in _load_itree_code_overrides()
Management command random_trees now takes an option for probability of diameter existing
Require species (rather than otm code) for benefits calculation

Now that we have ITreeCodeOverride's, species without OTM codes can contribute to benefits calculations
Merge pull request #955 from RickMohr/itree-override-performance

Fix performance issues when computing benefits with ITreeCodeOverride table
Add exit prompt when leaving unsaved changes

When leaving certain data entry modes, including but not limited to 'Add Tree' and 'Edit Tree', you can now lock the page so that the user will be prompted before leaving the page, or before leaving that particular edit mode.
Merge pull request #954 from steventlamb/topic/prompt_when_changing_modes

Add exit prompt when leaving unsaved changes
Allow non-empty falsy values for UDF choices, Fixes #950
Add USDA Grant info
Merge pull request #956 from maurizi/fix/950/allowZeroForUDFChoices

Allow non-empty falsy values for UDF choices, Fixes #950
Change text for add tree, Fixes #966
Don't prompt when leaving the page for add tree -> continue editing.
Fixes #969, Fixes #969
Merge pull request #979 from maurizi/fix/969/addTreePrompt

Don't prompt when leaving the page for add tree -> continue editing.
Merge pull request #977 from maurizi/fix/966/textChange

Change text for add tree, Fixes #966
plot -> planting site
Use more consistent test case names
Remove unused requirement
Fix bad delete url from tree detail page

Tree deletion was working from a plot detail url but broken from a tree detail url.
Allow 'administrator' role to delete objects