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.

main.go

@@ -101,36 +101,36 @@ type appContext struct {
 	adminUsers     []User
 	invalidTokens  []string
 	// Keeping jf name because I can't think of a better one
-	jf                   *mediabrowser.MediaBrowser
-	authJf               *mediabrowser.MediaBrowser
-	ombi                 *OmbiWrapper
-	js                   *JellyseerrWrapper
-	thirdPartyServices   []ThirdPartyService
-	datePattern          string
-	timePattern          string
-	storage              Storage
-	validator            Validator
-	email                *Emailer
-	telegram             *TelegramDaemon
-	discord              *DiscordDaemon
-	matrix               *MatrixDaemon
-	contactMethods       []ContactMethodLinker
-	info, debug, err     *logger.Logger
-	host                 string
-	port                 int
-	version              string
-	URLBase              string
-	updater              *Updater
-	newUpdate            bool // Whether whatever's in update is new.
-	tag                  Tag
-	update               Update
-	proxyEnabled         bool
-	proxyTransport       *http.Transport
-	proxyConfig          easyproxy.ProxyConfig
-	internalPWRs         map[string]InternalPWR
-	pwrCaptchas          map[string]Captcha
-	ConfirmationKeys     map[string]map[string]newUserDTO // Map of invite code to jwt to request
-	confirmationKeysLock sync.Mutex
+	jf                    *mediabrowser.MediaBrowser
+	authJf                *mediabrowser.MediaBrowser
+	ombi                  *OmbiWrapper
+	js                    *JellyseerrWrapper
+	thirdPartyServices    []ThirdPartyService
+	datePattern           string
+	timePattern           string
+	storage               Storage
+	validator             Validator
+	email                 *Emailer
+	telegram              *TelegramDaemon
+	discord               *DiscordDaemon
+	matrix                *MatrixDaemon
+	contactMethods        []ContactMethodLinker
+	info, debug, err      *logger.Logger
+	host                  string
+	port                  int
+	version               string
+	URLBase, ExternalHost string
+	updater               *Updater
+	newUpdate             bool // Whether whatever's in update is new.
+	tag                   Tag
+	update                Update
+	proxyEnabled          bool
+	proxyTransport        *http.Transport
+	proxyConfig           easyproxy.ProxyConfig
+	internalPWRs          map[string]InternalPWR
+	pwrCaptchas           map[string]Captcha
+	ConfirmationKeys      map[string]map[string]newUserDTO // Map of invite code to jwt to request
+	confirmationKeysLock  sync.Mutex
 }
 
 func generateSecret(length int) (string, error) {