Mitigated issue with non-nullable checkboxes

The `ImportOptions.DeleteUnmatched*` attributes are all `bool`, but are backed by a nullable `bool?` which defaults to `null` and inherits its value based on the `ImportOptions.Strategy` value. Unfortunately, when using the ASP.NET Core `input` tag helper with checkboxes, a hidden field is generated with the value of `false`. This results in the value of these properties _always_ being set to false, and no longer determing their value from the `ImportOptions.Strategy`. We only want that to happen when they're set to `true`. To mitigate that, we're removing the tag helper, and manually defining the `name` attribute. This introduces a risk in the case that the `enum` values change; to help avoid that potentiality in the future, we used `nameof()` on both the `enum` as well as its fields, so a compilation error will occur if those values are altered.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Areas/Editor/Views/Editor/Import.cshtml

@@ -147,22 +147,22 @@
             <h3>Delete Unmatched…?</h3>
             <section class="attribute">
               <label>
-                <input type="checkbox" asp-for="ImportOptions.DeleteUnmatchedAttributes" /> Attributes
+                <input type="checkbox" name="@nameof(ImportOptions).@nameof(ImportOptions.DeleteUnmatchedAttributes)" value="true" /> Attributes
                 <i class="fa fa-info-circle" data-toggle="tooltip" data-placement="right" title="Deletes existing attributes if they aren't included in the import. Implicit if the merge strategy is set to replace."></i>
               </label>
               <br />
               <label>
-                <input type="checkbox" asp-for="ImportOptions.DeleteUnmatchedRelationships" /> Relationships
+                <input type="checkbox" name="@nameof(ImportOptions).@nameof(ImportOptions.DeleteUnmatchedRelationships)" value="true" /> Relationships
                 <i class="fa fa-info-circle" data-toggle="tooltip" data-placement="right" title="Deletes existing relationships if they aren't included in the import. Implicit if the merge strategy is set to replace."></i>
               </label>
               <br />
               <label>
-                <input type="checkbox" asp-for="ImportOptions.DeleteUnmatchedNestedTopics" /> Nested Topics
+                <input type="checkbox" name="@nameof(ImportOptions).@nameof(ImportOptions.DeleteUnmatchedNestedTopics)" value="true" /> Nested Topics
                 <i class="fa fa-info-circle" data-toggle="tooltip" data-placement="right" title="Deletes existing nested topics if they aren't included in the import. Implicit if the merge strategy is set to replace."></i>
               </label>
               <br />
               <label>
-                <input type="checkbox" asp-for="ImportOptions.DeleteUnmatchedChildren" /> Children
+                <input type="checkbox" name="@nameof(ImportOptions).@nameof(ImportOptions.DeleteUnmatchedChildren)" value="true" /> Children
                 <i class="fa fa-info-circle" data-toggle="tooltip" data-placement="right" title="Deletes existing children—including nested topics—if they aren't included in the import. Implicit if the merge strategy is set to replace."></i>
               </label>
             </section>