Add "None" theme

This basically disables syntax highlighting. That doesn't mean we use
absolutely no colors - the search match, suggestion, selection and the
pager have coloring, but only reverse or brblack.

The idea is that this disables anything that tells you about
the *syntax*, but it still tells you about the state of the
commandline. If we didn't highlight the selection it would be entirely
invisible, and if we didn't highlight the suggestion you would have no
idea where it begins.

So this basically brings colors on-par with bash, where the search
match is colored (in reverse) and suggestions aren't a thing.

An alternative is to add a $fish_highlighting_enabled variable like
the one for suggestions. That's still possible, but would require some
internal changes to avoid coloring some things with $fish_color_normal
and other things with the normal terminal color.

One thing this also does not do is set the git prompt colors. These
are currently disallowed from being set in theme files because they
start with `__fish` instead of just `fish`. We should probably rename
them.

share/tools/web_config/themes/none.theme

@@ -0,0 +1,31 @@
+# name: '(Almost) No Colors'
+# preferred_background: 'black'
+
+fish_color_autosuggestion brblack
+fish_color_cancel
+fish_color_command
+fish_color_comment
+fish_color_cwd normal
+fish_color_cwd_root normal
+fish_color_end
+fish_color_error
+fish_color_escape
+fish_color_history_current
+fish_color_host normal
+fish_color_keyword
+fish_color_match
+fish_color_normal normal
+fish_color_operator
+fish_color_param
+fish_color_quote
+fish_color_redirection
+fish_color_search_match -r
+fish_color_selection -r
+fish_color_status normal
+fish_color_user normal
+fish_color_valid_path
+fish_pager_color_completion normal
+fish_pager_color_description brblack
+fish_pager_color_prefix '--underline'
+fish_pager_color_progress brblack
+fish_pager_color_selected_background --background=brblack