Add router.basePath config option for deployments at non-webroot

This is in preparation of configuring Vue with publicPath: "".

docs/config.md

@@ -63,6 +63,8 @@ window.owntracks.config = {};
   - [`fitView`](#onlocationchangefitview)
   - [`reloadHistory`](#onlocationchangereloadhistory)
 - [`primaryColor`](#primarycolor)
+- `router`
+  - [`basePath`](#routerbasepath)
 - [`selectedDevice`](#selecteddevice)
 - [`selectedUser`](#selecteduser)
 - [`showDistanceTravelled`](#showdistancetravelled)
@@ -435,6 +437,22 @@ Primary color for the user interface (navigation bar and various map elements).
   };
   ```
 
+### `router.basePath`
+
+Base path of the application deployment.
+
+- Type: [`String`]
+- Default: `"/"`
+- Example:
+  ```js
+  // Frontend will be reachable at https://example.com/owntracks
+  window.owntracks.config = {
+    router: {
+      basePath: "/owntracks",
+    },
+  };
+  ```
+
 ### `selectedDevice`
 
 Initial selected device. All devices will be shown by default if `null`. Will be ignored

src/config.js

@@ -71,6 +71,9 @@ const DEFAULT_CONFIG = {
     reloadHistory: false,
   },
   primaryColor: "#3f51b5",
+  router: {
+    basePath: "/",
+  },
   selectedDevice: null,
   selectedUser: null,
   showDistanceTravelled: true,

src/router.js

@@ -1,12 +1,13 @@
 import Vue from "vue";
 import Router from "vue-router";
-import Map from "./views/Map.vue";
+import config from "@/config";
+import Map from "@/views/Map.vue";
 
 Vue.use(Router);
 
 export default new Router({
   mode: "history",
-  base: process.env.BASE_URL,
+  base: config.router.basePath,
   routes: [
     {
       path: "/",