Fix duplicate download IDs.

Fixes https://github.com/profanity-im/profanity/issues/1794

Explanation
The problem is the download's identifier. Downloads are given an ID so they can be referenced later when their progress changes. Currently, the download's ID is the download's URL. When you download the same file twice, you have two downloads with the same ID. Download progress updates are shown on the first of both downloads with the same ID.

Solution
Change the download's ID from its URL to a random number. A random ID is generated when get_random_string() is called from cmd_funcs.c. Several other functions are updated to cope with the new ID format.

tests/unittests/tools/stub_aesgcm_download.c

@@ -8,6 +8,7 @@ typedef struct http_download_t HTTPDownload;
 
 typedef struct aesgcm_download_t
 {
+    char* id;
     char* url;
     char* filename;
     ProfWin* window;

tests/unittests/tools/stub_http_download.c

@@ -8,6 +8,7 @@ typedef struct prof_win_t ProfWin;
 
 typedef struct http_download_t
 {
+    char* id;
     char* url;
     char* filename;
     char* directory;

tests/unittests/ui/stub_ui.c

@@ -490,7 +490,7 @@ mucwin_unset_message_char(ProfMucWin* mucwin)
 
 void win_update_entry_message(ProfWin* window, const char* const id, const char* const message){};
 void win_mark_received(ProfWin* window, const char* const id){};
-void win_print_http_transfer(ProfWin* window, const char* const message, char* url){};
+void win_print_http_transfer(ProfWin* window, const char* const message, char* id){};
 void win_print_loading_history(ProfWin* window){};
 
 void