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.

ts/modules/settings.ts

@@ -33,6 +33,7 @@ interface Setting {
     depends_true?: string;
     depends_false?: string;
     wiki_link?: string;
+    deprecated?: boolean;
 
     asElement: () => HTMLElement;
     update: (s: Setting) => void;
@@ -579,6 +580,7 @@ class sectionPanel {
             if (name in this._settings) {
                 this._settings[name].update(setting);
             } else {
+                if (setting.deprecated) continue;
                 switch (setting.type) {
                     case "text":
                         setting = new DOMText(setting, this._sectionName, name);