accounts: add "record count", start searchable user cache

RecordCounter class created from that in activityList, and put in
accountsList. PageCount-type route standardized and made for /users
(/users/count). Created userCache, which regularly generates the
respUser list returned by /users. Added a currently dumb POST /users for
searching/pagination, GET /users is now just for getting -all- users.
go-getted expr, an expression language that seems like it'll be useful
for evaluating local searches. We don't store this data in the badger
    DB, so we can't use the nice query form provided by badgerhold.

api-activities.go

@@ -126,8 +126,8 @@ func (app *appContext) GetActivities(gc *gin.Context) {
 
 	resp := GetActivitiesRespDTO{
 		Activities: make([]ActivityDTO, len(results)),
-		LastPage:   len(results) != req.Limit,
 	}
+	resp.LastPage = len(results) != req.Limit
 
 	for i, act := range results {
 		resp.Activities[i] = ActivityDTO{
@@ -173,12 +173,12 @@ func (app *appContext) DeleteActivity(gc *gin.Context) {
 
 // @Summary Returns the total number of activities stored in the database.
 // @Produce json
-// @Success 200 {object} GetActivityCountDTO
+// @Success 200 {object} PageCountDTO
 // @Router /activity/count [get]
 // @Security Bearer
 // @tags Activity
 func (app *appContext) GetActivityCount(gc *gin.Context) {
-	resp := GetActivityCountDTO{}
+	resp := PageCountDTO{}
 	var err error
 	resp.Count, err = app.storage.db.Count(&Activity{}, &badgerhold.Query{})
 	if err != nil {