config: migrate "url_base" dupes to "jfa_url"

URL Base now refers to JUST the subfolder portion, i.e. `/accounts` if
you access jfa-go at `http://jellyf.in/accounts`. General > "jfa_url"/"External
jfa-go URL" now refers to the WHOLE URL you access jfa-go at, i.e.
`http://jellyf.in/accounts`. The settings in "invite emails" and
"password resets" have been removed, and a value chosen from the two
applied to "jfa_url". Migration also makes a config backup. Adds a
"deprecated" flag to config-base, which just tells the UI to not show
it (for now). Also added some warnings related to the URL base /
External URL.

models.go

@@ -229,6 +229,7 @@ type setting struct {
 	DependsTrue     string      `json:"depends_true,omitempty"`  // If specified, this field is enabled when the specified bool setting is enabled.
 	DependsFalse    string      `json:"depends_false,omitempty"` // If specified, opposite behaviour of DependsTrue.
 	Style           string      `json:"style,omitempty"`
+	Deprecated      bool        `json:"deprecated,omitempty"`
 }
 
 type section struct {