Remove unused `using`s from 3db0a803062df19e83bdd62b63eba47c016af384 change
- see 3db0a803062df19e83bdd62b63eba47c016af384
Introducing RazorTextWriter

RazorTextWriter represents the result of rendering a page as a sequence of
strings rather than a concatenated string. This avoids building up large
strings in memory.
Updating project.json to use frameworks token
Correct XML comment syntax
- fix problems at least with Roslyn compiler and VS IntelliSense or the Object Browser
 - `<see langref="keyword"/>` generates nothing
 - `<example>` at top level (outside `<summary>`) generates nothing
 - curly braces don't become angle brackets outside `<see cref="reference"/>` references
  - yeah, a point @yishaigalatzer asked about in a previous PR
- `<see href="reference"/>` is not valid
- correct some invalid use of angle brackets and remove useless empty elements
- correct unresolved XML comment references; generally, add namespace prefix

Symptoms for some of the above issues included
- generated XML comments such as `<!-- Badly formed XML comment ... -->`, usually indicating an unclosed element
- generated XML attributes such as `cref="!:..."`, indicating a broken reference
 - in a couple of cases we had `<typeparamref cref="TOption"/>`; attribute should be `"name"`

Few wording changes beyond
- `"opening </form> tag"` -> `"<form> start tag"`
- `"closing </form> tag"` -> `"</form> end tag"`

Also correct two typos in `HtmlHelper`

Will create a unit test to ensure XML syntax doesn't degrade going forward.  Separate PR.
- for now, check using `dir -r *.xml | sls '!'`
Updating release Nuget.config
Updating dev Nuget.config
Make HTML helper `null` handling consistent
- #874 lines 3, 4, and 6
- correct `Value()` to treat a `null` expression name the same as `string.Empty`
- add missing `[NotNull]` attributes in `EditorExtensions` and for `GenerateIdFromName()`
- consistently pass `null` for default expression names to the helpers
 - for example, from extension methods
- add test cases using `null` for expression name

nits:
- correct summary XML comment for `HtmlHelper` class
- use named parameters and prefer interface (not extension) methods in changed calls
- use `string.Empty` instead of `""` in a few tests
Call `IHtmlHelper` methods from extension methods
- don't call other extension methods
- #847 line 7
- remove extension methods that do nothing but pass through to the interface method
 - weren't exactly ambiguous (interface method wins) but were useless
Prefer `IHtmlHelper` methods elsewhere in our code
- #847 line 7 sub-bullets
- don't call extension methods from our templates
 - partially addressed in PR #934 (for methods changed there)
- don't reference extension methods from XML comments
Correct extension class names to match containing files
- #874 line 5
- `EditorExtensions` -> `HtmlHelperEditorExtensions`
- `SelectExtensions` -> `HtmlHelperSelectExtensions`
Remove incorrect references to `HtmlHelper` class
- extension methods extend `IHtmlHelper`
Routing related fixes for #55
Fix Challenge params order.
Correct gaps in PR #934
- correct XML comment typo introduced in `HtmlHelper` in 56d66c090eaedc33e4838437e7611ba2b9ca8929 (bad merge)
- fix missed `null` requirement for `@Html.RadioButtonFor()` and remove buried `null` check
- add back `Environment.Newline` to `@Html.TextArea()` (was dropped though comment wasn't)
Simplify some XML comment references
- follow VS fixup suggestion and change e.g. `<see cref="System.String"/>` -> `<see cref="string"/>`