feat: make expression persistent in the overpass importer

In other words: fill in the "tags" input with the latest
typed expression.

Otherwise, when making tries with overpass imports, one have to
type it again and again (it's available through the autocomplete,
though, but it's not obvious and it's less friendly IMHO).

umap/static/umap/js/modules/importers/overpass.js

@@ -53,11 +53,15 @@ export class Importer {
       'https://photon.komoot.io/api?q={q}&layer=county&layer=city&layer=state'
     this.id = 'overpass'
     this.boundaryChoice = null
+    this.expression = null
   }
 
   async open(importer) {
     const container = DomUtil.create('div')
     container.innerHTML = TEMPLATE
+    if (this.expression) {
+      container.querySelector('[name=tags]').value = this.expression
+    }
     this.autocomplete = new Autocomplete(container.querySelector('#area'), {
       url: this.searchUrl,
       placeholder: translate(
@@ -80,6 +84,7 @@ export class Importer {
         Alert.error(translate('Expression is empty'))
         return
       }
+      this.expression = form.tags
       let tags = form.tags
       if (!tags.startsWith('[')) tags = `[${tags}]`
       let area = '{south},{west},{north},{east}'